Sharing a wide range of experiences and perspectives relating to mineralogy as a hobby

Thursday, February 8, 2018

Alfredo Petrov and an Opal Sulfur Dilemma

Something is different in Tucson this year. Alfredo Petrov and his inventory of mostly unusual specimens with precise handwritten labels are nowhere to be found at the Inn Suites. Nor will he be doing the big show. Instead, he is among a small group of wholesale and retail dealers with interesting minerals in the JTI tent at La Fuente de Piedras, 1535 North Oracle.

Here as at the Inn Suites, Alfredo holds forth about minerals with a diverse stream of connoisseurs, cognoscenti, and the simply curious. More often than not, the topic relates to specimens he is selling. It seemed odd when he picked up from amongst those specimens a small cabinet specimen of native sulfur crystals. Their locality was the El Desierto Mine in southwestern Bolivia. These crystals seemed at first an unlikely pick for Alfredo, a dealer of minerals that are less common. Nearby, a Jewel Tunnel Imports table was completely covered with similar pieces.

Alfredo acknowledged that that it was the bright yellow color of these and all native sulfur crystals that attracted most collectors. Except for certain uranium bearing minerals, not many easily acquired species can compete with them in terms of brilliance. He recalls when his mentor, the late mineralogical icon Rock Currier instructed him: "Alfredo, I'm going to teach you the three secrets to success at selling minerals. They are #1 color; #2 color; #3 color." Interestingly, Currier was the first to bring El Desierto Mine sulfur crystals to the US. He imported about 15 tons of them. Over the last decade, other dealers have imported another five or so tons. The market for these beautiful crystals is endless.

Alfredo's interest, however, goes beyond the color. He points to the white transparent material atop some of the crystals. It is hyalite opal. Viewed from a distance, its presence makes the crystals seem less distinct, even less brilliant. Alfredo explains:

"What's important is that the sulfur forms from volcanic fumarole fumes. The sulfur is in the gas, and the crystals grow out of the gas. There is no liquid involved as there is when most minerals form. If the hyalite opal formed volcanic gases, where could the silica be in that gas? Opal is derived from quartz, which doesn't evaporate, so you don't get siliceous gas.

He continues:

"The melting point of sulfur is 108°C. The presence of the hyalite proves that the temperature of opal deposition. was 108°C or less. If the opal had ever reached that temperature, the sulfur crystals would have been melted. "

Alfredo then points to a larger specimen where a small amount of the sulfur is black in color. He notes that analysis has shown the black material to be covellite, a copper sulfide. This occurrence raises a similar question. The type locality for covellite is Mount Vesuvius, which demonstrates its potential for volcanic origin.

Metallic sulfides formed as volcanic sublimates are rare albeit explainable. But how does one explain the presence of silica in any any kind of volcanic sublimate? Despite his legendary knowledge of minerals, Alfredo does not have a sure answer.

"There is a well-known theory among volcanologist," he notes, "that silica is trasported by HF (hydrofluoric acid), a very, very, very nasty gas, which transports silicon as SiF4 (silicon tetrafluoride)."

Alfredo spends considerable time field collecting in volcanic regions, especially in Japan. And then:

"If this is true, then spending much time breathing around volcanic fumaroles would be a very unhealthy experience, and I'm going to die a nasty death."

Let's hope it isn't true, and there's a different chemical mechanism we don't know about yet.